1. Field of the Invention
The present invention relates to a spread illuminating apparatus as a lighting means for a liquid crystal display apparatus, and the like, and more particularly to a side-light type spread illuminating apparatus using a spot light source.
2. Description of the Related Art
Recently, a liquid crystal display (LCD) apparatus is extensively used as a display device for electronic equipments, such as a personal computer, a cellular phone, and the like. Unlike a cathode-ray tube (CRT), a liquid crystal does not emit light by itself, and therefore, for example, a light transmission type LCD apparatus requires a lighting means to illuminate a liquid crystal panel from behind, and a semi-transmission type LCD apparatus, which utilizes outside light, requires an auxiliary lighting means when used in the dark. A side-light type spread illuminating apparatus, which generally includes a light conductive plate and a light source disposed at a side surface of the light conductive plate, is advantageous in achieving a low profile, and therefore is preferably used as a lighting means for an LCD apparatus. In such a side-light type spread illuminating apparatus, a line light source, such as a cold-cathode discharge lamp, is conventionally used.
Under the circumstances, recently, a white light emitting diode comes up with an enhanced performance, and is increasingly employed in place of the line light source for purpose of achieving a smaller dimension, a lower profile, and a reduced power consumption.
FIG. 5 shows a spread illuminating apparatus 100 which generally includes a light conductive plate 101, a plurality (three in the figure) of light emitting diodes (LED's) 102, and a prism sheet 103. The spread illuminating apparatus further includes a reflection plate for the LED's 102, a reflection plate disposed under the light conductive plate 101, and a housing frame, but these components are omitted in FIG. 5.
The light conductive plate 101 is a plate-like member formed of a transparent resin material, such as methacrylic resin, polycarbonate resin, or the like, defines a light exit surface 101a, a light refection surface 101b opposite to the light exit surface 101a, and a light entrance surface 101c. The light exit surface 101a has a plurality of triangular prisms extending orthogonally to the light entrance surface 101c and arrayed at a constant interval, and the light reflection surface 101b has a light reflecting means to regularly or irregularly reflect light toward the light exit surface 101a so that at least some part of the reflected light impinges on the light exit surface 101a at an incidence angle smaller than a critical angle.
The prism sheet 103 is a sheet-like member formed of a transparent resin material, for example, a polyethylene terephthalate (PET) film, and defines a major surface 103a having a plurality of triangular prisms formed of methacrylic resin, polycarbonate resin, or the like, and extending in one direction The prism sheet 103 is disposed on the light conductive plate 101 such that the major surface 103a opposes the light exit surface 101a with their respective triangular prism arrays extending orthogonally to each other.
In the spread illuminating apparatus 100, a primary light source is constituted by an array of the plurality of LED's 102 which are spot lights. Such a light source, unlike a line light source adapted to emit light with a relatively uniform brightness across its longitudinal direction, involves a non-uniform brightness distribution with respect to its array direction with brightness peaks appearing in front of respective LED's 102. As a result, light exiting from the spread illuminating apparatus 100 suffers an uneven brightness attributed to the non-uniform brightness distribution, unless some measures are taken to deal with the non-uniformity.
In the spread illuminating apparatus 100 using one piece of prism sheet (the prism sheet 103) in FIG. 5, when a viewer observes the light conductive plate 101 from the side of the light exit surface 101a, light is viewed as radiated in a substantially uniform manner on the whole. However, it happens that emission lines H configuring a V-letter appear at respective LED's 102 as shown in FIG. 6 (schematically illustrated with exaggeration), which results in a significantly deteriorated lighting quality. Such a problem occurs when a regular reflection means, for example, a prism projection pattern, is provided on the front or rear major surface of the light conductive plate 101, while it does not happen when an irregular reflection means, for example, dotted projection pattern, is provided
The emission lines H appear due to the LED's 102 used as a light source in place of a cold-cathode discharge lamp. The mechanism of development of the emission lines H is not definitely found out yet, but the problem is attributed to the LED's 102 having a directivity, and to that light traveling in the light conductive plate 101 is apt to be reflected at prism projections located close to the light entrance surface 101c so as to proceed in a specific direction toward the light exit surface 101a. This is pointed out in, for example, Japanese Patent Application Laid-Open No. 2004-6187, paragraphs [0006], and [0070] through [0073].
In order to deal with the problem described above, the aforementioned Japanese Patent Application Laid-Open No. 2004-6187 discloses a spread illuminating apparatus, in which a light reflection surface of a light conductive plate has a plain flat area covering a certain distance from a light entrance surface and then has a prism structure with its prism protrusion dimension varying according to the distance from the light entrance surface. However, fabricating such a light reflection surface is troublesome taking a lot of time and work thus pushing up the cost.